Welcome to iDVD Discussions.
That may be a media/burning speed issue.
I recommend a disk image burned to Verbatim DVD-R at 4X or slower using Toast or Disk Utility.
David Pogue also recommends Verbatim on page 356 of his latest book:
iMovie HD & iDVD 6: The Missing Manual
Quoting David: "Cheaper brands don't use the same amount of organic dyes and are more likely to suffer premature deaths."
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Suggest you create a disc image and then burn the DVD. File/Save as Disc Image...
http://docs.info.apple.com/article.html?path=iDVD/6.0/en/18.html
http://www.kenstone.net/fcphomepage/image_to_diskstone.html
This will isolate any encoding/burning issues you may encounter. Once the disc image is created, double-click the .img and burn the virtual disc that should appear on your desktop, using Toast to burn the DVD. Disk Utility to burn the .img file. Usually, you can select a burning speed in Disk Utility.
There are variations to this process based on which OS X you are using...
Open Disk Utility (in Utilities folder in Applications folder), click on the virtual disc (maybe the .img) in the left-hand window. Click the Burn icon. A new window should drop down and your SuperDrive tray will open after clicking the Burn icon. Insert a recordable DVD. (Verbatim DVD-R preferred by me.) Click the Close button. Wait. Select a burn speed. If you hold your mouse cursor over the pop-up it says: "Select a slower speed to work around burn failures," so select 4x or slower for best results. Then click the Burn button.
-->If the virtual disk selection won't allow you to click the Burn icon, use the .img file instead. This may have changed in 10.3.9 and did change in Tiger.
Also, you can use DVD Player to play the virtual disk to check your iDVD project before burning to DVD. Launch DVD Player. File/Open VIDEO_TS (Open DVD Media... in Player 4.6). Find the VIDEO_TS folder and open that. (The audio folder is for DVD-Audio disks.)
http://docs.info.apple.com/article.html?artnum=93006
